There has been considerable recent interest in developing CO.sub.2 slab lasers. These lasers have been shown to generate high output powers in an efficient manner. Prior art references discussing such slab lasers include "Radio-frequency excited Stripline CO and CO.sub.2 lasers," Gabai, Hertzberg and Yatsiv, Abstract presented at Conference on Lasers and Electro-optics, June 1984; U.S. Pat. No. 4,719,639, issued Jan. 12, 1988 to Tulip; U.S. Pat. No. 4,939,738 issued Jul. 3, 1990 to Opower and U.S. Pat. No. 5,048,048 issued Sep. 10, 1991 to Nishimae.
The assignee herein has developed a CO.sub.2 slab laser which is described in a copending patent application Ser. No. 07/596,788, filed Oct. 12, 1990 now U.S. Pat. 5,38 herein by reference. The laser described therein includes a pair of spaced electrodes configured to define a rectangular discharge region. The lasing gas is excited by passing an RF current through the electrodes. A pair of mirrors are mounted at the ends of the electrodes to define the resonator.
In the preferred embodiment of the latter laser, the spacing between the electrodes is selected so that light is guided between the surfaces of the electrodes. In the wider dimension, the light propagates in free space and is confined by the resonator mirrors. As described in the above cited application, to maximize performance, the resonator mirrors are spherical and selected to define a stable resonant cavity along the waveguide axis (the axis extending between the electrodes) and an unstable resonator perpendicular thereto (the free space axis). In addition, the spacing between the end of the electrodes and the mirrors is selected so that the radius of curvature of the wavefront of the laser beam in the waveguide axis at the mirror location matches the radius of curvature of the mirrors selected for the unstable resonator. Since the mirrors are spaced from the end of the electrodes, the gas discharge tends to extend out to the very ends of the electrodes.
When this laser has been life tested at powers exceeding 150 Watts, the output power has begun to diminish after only a few hundred hours. This decrease in output power has been traced to the deterioration of the mirrors. As previously disclosed in the above cited application, mirror degradation at lower powers had been addressed by adding a very thin coating of thorium fluoride to the top surface of the mirror. Thorium fluoride is less reactive and helped to reduce the level of degradation of the mirrors. However, at higher powers, this protective overcoating has proved insufficient and the mirrors are still the primary lifetime limiting component of the laser.
An analysis of mirrors which have shown degradation at higher powers indicates that the problem arises due to an initial oxidation of the mirror coating. The oxidized layer has a much higher absorption loss than the coating in its initial state. This added loss leads to a power decline in the laser.
Oxidizing species are known to be generated in the gas discharge between the two electrodes. The species include oxygen atoms, ozone, excited oxygen molecules and possibly ions of either carbon monoxide, carbon dioxide or oxygen. Some of these species will escape the confines of the slab and begin diffusing away from the discharge region and towards the mirrors. Since the discharge region extends to the end of the electrodes, the possibility of the oxidizing species migrating to the mirrors is increased.
Accordingly, it is an object of the subject invention to reduce the effects of the discharge on the resonator mirrors.
It is a further object of the subject invention to control the amount of oxidizing species reaching the resonator mirrors.
It is another object of the subject invention to enhance the lifetime and improve the performance of a slab laser.